The present invention relates to (i) the use of octahedral transition metal complexes as precatalysts for the polymerization of alpha-olefins; (ii) the use of homogeneous catalyst systems comprising these complexes and (iii) a novel class of such complexes. More particularly, the invention relates to the use of cationic chiral, racemic or non-chiral catalysts of the above-mentioned type for the polymerization of alpha-olefins to produce highly stereoregular polymers or poly alpha-olefin elastomers.
The polymerization of alpha-olefins with Ziegler-Natta catalysts is well-known in the chemical industry and it is used to a large extent. The various polymers that are derived from the polymerization of such olefins show differences in their chemical and physical properties, as a result of differences in molecular structure and molecular weights. Polymers of alpha-olefins having three or more carbon atoms as the monomeric unit will have pendant hydrocarbyl groups attached to the polymer backbone chain. The arrangement of these hydrocarbyl groups along the polymer backbone will determine, to a large extent, the physical properties of a particular polymer. For example, strong polymers tend to be stereochemically regular meaning that the adjacent pendant hydrocarbyl groups reside on the same side of the polymer backbone.
Three major types of stereoregularity, or tacticity have been characterized: atactic, isotactic and syndiotactic configurations. Atactic polyolefins are those wherein the pendant hydrocarbyl groups have no regular order in space with reference to the backbone and to other pendant groups. These are amorphous materials and are generally unsuitable for applications where high strength is required. Isotactic polyolefins are those wherein the pendant hydrocarbyl groups are ordered in space on the same side or plane of the polymer backbone chain. Polyolefins that are highly isotactic exhibit a high degree of crystallinity and high melting points, and accordingly are particularly suited to high strength applications.
The degree of stereoregularity may be determined from the pentad analysis using carbon-NMR techniques. A purely isotactic polyolefin will have a degree of crystallinity (mmmm) value of close to 100, whereas atactic poly alpha-olefins will have a theoretical statistical value of 6.5.
Syndiotactic polyolefins are those wherein the pendant hydrocarbyl groups of the polymer backbone alternate sequentially from one side or plane to the opposite side or plane relative to the polymer backbone. Although syndiotactic polymers, as compared with the corresponding isotactic polymers, are characterized by lower melting points, they are generally suitable for high strength applications, provided that their molecular weight exceeds 100,000 daltons.
Another category of high polymers with special stereosequencing is elastic polymers, or elastomers. As taught by Textbook of Polymer Science (edited by Fred W. Billmeyer, Jr., 3rd edition, Wiley Interscience: New York, 1984), page 507, the unique properties of elastomers include their ability to stretch and retract rapidly. Elastomers exhibit high strength and modulus while stretched, and recover fully on release of the stress. To obtain these properties, certain requirements are placed upon the molecular structure of the compounds: they must be high polymers, be above their glass transition temperatures, be amorphous in the unstretched state, but preferably develop crystallinity on stretching, and contain a network of crosslinks to restrain gross mobility of the chains.
In addition to the above-mentioned form of elastic polymers, polymers that contain crystalline areas (domains) and amorphous areas also exhibit elastomeric properties. This can be achieved in different ways, leading to (at least) two different microstructures, both of which are germane to the instant invention:
(a) a polymer containing domains of stereoregular and atactic sections, consecutively arranged in each polymer chain; and
(b) a stereoregular polymer in which the inclusion of numerous stereoregular mistakes provides the material with atactic parts.
The polymer described in (b) has elastomeric properties because the crystalline, stereoregular parts of the chains form lamellae, and the mistakes, incorporated into the otherwise stereoregular sequences, and when present in sufficient quantity, form the amorphous domains that provide the material with elastomeric properties.
One of the most important parameters to measure the elasticity is the modulus (for elongation of 300-400%). For example, natural rubber and SBR polymers have a modulus of 2500 psi; neoprene, which is not very elastic, has a modulus of about 1000 psi; isotactic polypropylene has a modulus of 91 psi.
Conventional titanium and zirconium based Ziegler-Natta catalysts for the preparation of isotactic polymers are well known in the art. The systems are, however, limited in terms of molecular weight, molecular weight distribution and tacticity control. Additional methods of producing isotactic polymers from an alumoxane cocatalyzed metallocene were reported by Ewen, J. Am. Chem. Soc., 106, 6355 (1984) and Kaminsky et al., Angew. Chem. Int. Ed. Eng., 24, 507 (1985).
The use of cocatalyzed catalyst systems for the production of highly crystalline polyolefins is disclosed in U.S. Pat. No. 5,318,935. The catalyst systems described therein comprise a complex formed upon admixture of the amido group IVb transition metal component with an alumoxane component.
According to a recent review (M. S. Eisen et al., J. Organometallic Chem., 503, 307 (1995)), a series of bis(trimethylsilyl)benzamidinate zirconium dichlorides are described as active catalysts for ethylene polymerization. As taught therein, the polymerization activity increases drastically with increasing pressure. However, these catalyst systems are generally characterized by a pronounced moisture-sensitivity due to the inherent hydrolytic instability imposed by the presence of several Sixe2x80x94N bonds in the molecules.
Homogeneous catalysts for stereoregular olefin polymerization are further disclosed in U.S. Pat. No. 5,330,948. According to this patent, by using a metallocene catalyst having a chiral substituent selected from neomenthyl, menthyl and phenylmenthyl with a cocatalyst, better control over the desired properties of the resulting polymer is achieved.
U.S. Pat. No. 5,594,080 describes metallocene catalysts bearing cyclopentadienyl-type ligands, which are used in the production of elastomeric polyolefins. The structure and therefore the properties of the obtained products depend on several factors, inter alia the olefin monomer pressure during the polymerization and the nature of the cyclopentadienyl-based ligands.
The synthesis of stereoregular polymers has been reported (M. Bochmann, J. Chem. Soc., Dalton Trans. 225, (1996); H. H. Brintzinger, D. Fischer, R. Mxc3xcllhaupt, B. Rieger and R. M. Waymouth, Angew. Chem., Int. Ed. Engl. 34, 1143 (1995)) by using chiral organo-group IV (Ti, Zr, Hf) catalysts having approximate C2 symmetry. Most of the ligands for these xe2x80x9cC2xe2x80x9d catalysts are based upon indenyl or related cyclopentadienyl components and are difficult and expensive to synthesize.
Octahedral transition-metal complexes for use in homogeneous catalyst systems for the polymerization of alpha-olefins are known. EP-A-0 675138 discloses the use of a catalyst comprising the cationic form of benzamidinato octahedral transition metal complexes and an anion of a Lewis acid or Brxc3x6nsted acid for the polymerization of alpha-olefins including propylene. The polymerization reaction can be carried out at a pressure in the range from atmospheric pressure to 200 kg/cm2G. Although the polymerization catalyst disclosed is a cationic form of an octahedral transition metal complex, and the homopolymerization of propylene is specifically disclosed, there is no mention of the tacticity of the polymer, and more specifically, there is no teaching of the tacticity characteristics of the polymer as a function of the olefin monomer or pressure.
EP-A-0 687693 describes the use of amidinato catalyst systems for the polymerization of alpha-olefins. Among possible cocatalysts, alumoxanes and boron compounds are mentioned. The alpha-olefins to be polymerized have at least 3 carbon atoms, and the polymerization can be carried out at a pressure of from 1 to 3000 bar. As with EP-A-0 675138, there is no indication of the tacticity characteristics of the polymer, nor is it suggested how the tacticity characteristics of the polymer may depend on the pressure or partial pressure of the olefin monomer. There is certainly no indication that the catalyst systems and operating conditions can be designed to produce polymers and copolymers having specific, desired tacticities. Furthermore, the formation of elastomers from homogeneous catalyst systems of this type is unknown.
There is thus a great need for, and it is highly desirous to have, inexpensive, moisture-insensitive catalyst systems that enable the production of stereoregular poly alpha-olefins in an efficient manner, with high conversions and yields, and a need for efficient methods of producing these stereoregular poly alpha-olefins. There is also a great need for catalyst systems and methods for the production of elastomeric poly alpha-olefins. Moreover, it would be highly advantageous to have catalyst systems and methods for the production of poly alpha-olefins in which the tacticityxe2x80x94qualitative and quantitativexe2x80x94of the polymers could be predicted and produced using rational design.
Accordingly, it is an object of the present invention to provide a method of polymerization of one or more alpha-olefins to form polymers having a preselected range of properties, ranging from isotactic to elastomeric properties.
It is another object of the invention to provide improved, low-cost homogeneous precatalysts for polymerization, which, when used with an appropriate cocatalyst, permit the efficient polymerization of alpha-olefins to produce highly stereoregular polymers.
It is another object of the invention to provide improved, low-cost homogeneous precatalysts for polymerization, which, when used with an appropriate cocatalyst, permit the efficient polymerization of alpha-olefins to produce elastomeric polymers.
It is another object of the invention to provide a new class of catalysts, which, as contrasted to the known catalysts that contain cyclopentadienyl ligands, are not decomposed when exposed to air or humidity.
Thus, according to the teachings of the present invention there is provided a process for the polymerization of one or more alpha-olefins having at least 3 carbon atoms, the process comprising the steps of (a) contacting the monomer or monomers in a solvent selected from the group consisting of polar and non-polar solvents under polymerization conditions with a homogeneous catalyst system including (i) a cationic form of a chiral or a non chiral octahedral transition metal complex, comprising 1, 2 or 3 bidentate chelating ligands and no cyclopentadienyl ligands and having symmetry selected from the group consisting of C1, C2, and C3 symmetries; and (ii) an anion of an acid selected from the group consisting of a Lewis acid and a Brxc3x6nsted acid; and (b) adjusting the pressure to obtain either a highly stereoregular polymer, a copolymer or an elastomer.
The homogeneous catalyst system can include a racemic or non-racemic mixture of said chiral octahedral transition metal complex.
According to the teachings of the present invention there is further provided a homogeneous catalyst system comprising (a) a cationic form of a chiral octahedral transition metal complex comprising 1, 2 or 3 bidentate chelating ligands and no cyclopentadienyl ligands and having symmetry selected from the group consisting of C1, C2, and C3 symmetries; and (b) an anion of an acid selected from the group consisting of a Lewis acid and a Brxc3x6nsted acid.
In a preferred embodiment, the symmetry of the homogeneous catalyst system is C1 symmetry. In another preferred embodiment, the symmetry of the homogeneous catalyst system is C2 symmetry. In yet another preferred embodiment, the symmetry of the homogeneous catalyst system is C3 symmetry.
In a preferred embodiment, the cationic form of the transition metal component of the catalyst system utilized in the above-mentioned process can be represented by formula A or formula B: 
wherein X is an anionic ligand; M is a transition metal atom selected from groups III, IV, and V of the periodic table of elements; B, the valency of M, equals 3, 4 or 5; Y is C, N, S, P, B or Si; Q1, Q2, T1 and T2 are each independently selected from the group consisting of O, OR, N, NR, NR2, CR, CR2, S, SR, SiR2, B, BR, BR2, P, PR and PR2, where each R is independently H or a group containing C, Si, N, O, B and/or P, whereas one or more R groups may be attached to M, each replacing the anionic ligand identified herein by X; Z is selected from H, OR, NR2, CR, CR2, CR3, SR, SiR3, PR2 or BR2; S is a solvent molecule; m is an integer selected from the group consisting of 1, 2, 3 or 4; n is an integer selected from the group consisting of 1, 2 or 3; y equals (B-n-1); and p is 0 or an integer to satisfy octahedral coordination requirements of the transition metal atom M.
In a preferred embodiment, the octahedral transition metal complexes characterized by formula A and formula B are of C1 symmetry, n equals one, and the transition metal is selected from the group consisting of zirconium and hafnium. The application of high pressure produces poly alpha-olefins that are substantially elastomeric.
In another preferred embodiment, the octahedral transition metal complex has C2 or C3 symmetry and the transition metal is selected from the group consisting of zirconium and hafnium. At high pressure, substantially pure isotactic poly alpha-olefins are obtained. At low pressure, non-stereoregular (atactic-type) poly alpha-olefins are obtained. When the pressure is alternated between high pressure and low pressure, an elastomer with atactic and isotactic segments is produced. The pressure alternation is preferably performed at a frequency lower than or about equal to the polymerization insertion rate. In a preferred embodiment, the alternation of pressure is carried out at a frequency of about 10xe2x88x924 to 100 Hertz.
Homogeneous catalyst systems for the production of stereoregular poly alpha olefins that include a cationic form of a chiral octahedral transition metal complex or a racemic mixture of an octahedral transition metal complex were not known prior to their disclosure in Israel Patent Application Ser. No. 121402 and Israel Patent Application Ser. No. 122105, from which the instant application claims priority. Moreover, according to known theory, a homogeneous catalyst system including a cationic form of a racemic mixture of an octahedral transition metal complex having C1 symmetry should produce isotactic (stereoregular) alpha-olefins; a non-chiral octahedral transition metal complex having C2v symmetry (or higher) should produce atactic alpha-olefins. By contrast, it has been discovered by the inventors of the instant invention that, at high pressures, homogeneous catalyst systems (of the present invention) which include a cationic form of octahedral zirconium or octahedral hafnium metal complexes having C1 symmetry and one bidentate chelating ligand, produce elastomers.
In a preferred embodiment, the transition metal of the octahedral complex is selected from the group consisting of vanadium, niobium, tantalum, scandium, yttrium, lanthanum, lanthanides, and actinides.
In this respect, the behavior of titanium would be expected to be qualitatively similar to that of zirconium and hafnium, because titanium, zirconium and hafnium belong to the same transition metal family and because they have been used in a substantially identical fashion in the polymerization of alpha-olefins with conventional Ziegler-Natta catalysts. It has been discovered, however, that at high pressure, homogeneous catalyst systems that include a cationic form of (chiral and non-chiral) octahedral titanium metal complexes of the present invention produce elastomers. The formation of elastomers using said octahedral titanium metal complexes is substantially irrespective of symmetry (C1, C2, and C3) and solvent. Alternation of pressure is unnecessary, such that operation is greatly simplified.
Hence, further according to the teachings of the present invention there is provided a process for the polymerization of one or more alpha-olefins having at least 3 carbon atoms, comprising contacting alpha-olefin monomer or monomers in a solvent selected from the group consisting of polar solvents and non-polar solvents under polymerization conditions and above atmospheric pressure with a homogeneous catalyst system, said homogeneous catalyst system including: (i) a cationic form of a chiral or a non chiral octahedral titanium complex, comprising 1, 2 or 3 bidentate chelating ligands and no cyclopentadienyl ligands; and (ii) an anion of an acid selected from the group consisting of a Lewis acid and a Brxc3x6nsted acid, to obtain an elastomer.
In a preferred embodiment, the homogeneous catalyst system includes a racemic or non-racemic mixture of said chiral octahedral titanium complex. Further provided is a homogeneous catalyst system including: (a) a cationic form of an octahedral titanium complex having CS symmetry, and (b) an anion of an acid selected from the group consisting of a Lewis acid and a Brxc3x6nsted acid.
Further provided is a process for the polymerization of one or more alpha-olefins having at least 3 carbon atoms, comprising contacting alpha-olefin monomer or monomers in a solvent selected from the group consisting of a polar solvent and a non-polar solvent under polymerization conditions, at or above atmospheric pressure, with a homogeneous catalyst system, said homogeneous catalyst system including: (i) a cationic form of an octahedral titanium complex having Cs symmetry; and (ii) an anion of an acid selected from the group consisting of a Lewis acid and a Brxc3x6nsted acid, to obtain an elastomer.
It has been discovered by the inventors of the instant invention that a chiral, octahedral transition metal complex having C1 or C2 symmetry is used to obtain syndiotactic polystyrene of very high stereoregularity. The activity of such complexes is over an order of magnitude higher than the activities of non-chiral octahedral complexes of the prior art (2.5xc3x97106-1xc3x97107 in the instant invention vs. 1.2xc3x97105-5.5xc3x97105). The stereoregularity of the polystyrene produced in processes of the instant invention is also superior to that of prior-art processes.
Hence, further provided is a polymerization process comprising the steps of contacting styrene monomer or monomers comprising styrene in a solvent selected from the group consisting of polar and non-polar solvents under polymerization conditions and at a pressure of about atmospheric or higher with a homogeneous catalyst system including: (i) a cationic form of a chiral octahedral transition metal complex comprising 1, 2 or 3 bidentate chelating ligands and no cyclopentadienyl ligands and having symmetry selected from the group consisting of C1 and C2 symmetries, and (ii) an anion of an acid selected from the group consisting of a Lewis acid and a Brxc3x6nsted acid, to obtain syndiotactic polystyrene or a copolymer comprising syndiotactic polystyrene.
In a preferred embodiment of the invention, an octahedral complex having C1 or C2 symmetry is used at low pressure to obtain an elastomeric copolymer containing sequences of syndiotactic polystyrene and atactic poly-aliphatic alpha-olefins. Propylene is a preferred aliphatic alpha-olefin.
In another preferred embodiment, an octahedral complex having C1 or C2 symmetry is used at alternating pressure to obtain an elastomeric copolymer containing sequences of syndiotactic polystyrene and isotactic and atactic sequences of poly-aliphatic alpha-olefins. Propylene is a preferred aliphatic alpha-olefin for this embodiment.
It has been found that homogeneous catalyst systems having cationic octahedral transition metal complexes (of the present invention) and possessing C1, C2 or C3 symmetry are suitable for hydrogenation reactions. Racemic mixtures of C2-symmetry complexes and monoheteroallylic complexes have been found to be particularly suitable.
As described above, the present invention largely relates to homogeneous catalyst systems having cationic octahedral transition metal complexes and to processes utilizing such systems to produce materials of desired tacticity: stereoregular polymers or elastomeric polymers. By sharp contrast, according to known theory and experimentation, tetrahedral transition metal complexes produce atactic polymers.
Surprisingly, it has been discovered that tetrahedral transition metal complexes of a particular structure can produce elastomeric polymers. Moreover, the elastomeric polymers are of a novel and significant variety.
Hence, a process is provided for the polymerization of alpha-olefins having at least 3 carbon atoms is herein provided, comprising contacting the alpha-olefin monomer in a solvent selected from the group consisting of polar solvents and non-polar solvents under polymerization conditions, at or above atmospheric pressure, with a homogeneous catalyst system comprising (a) a cationic form of a tetrahedral complex having (i) a transition metal center, and (ii) one or more Lewis basic pendant groups capable of donating one or more pairs of free electrons to the transition metal center; and (b) an anion of an acid selected from the group consisting of a Lewis acid and a Brxc3x6nsted acid, to obtain an elastomer.
In a preferred embodiment, the cationic form of the tetrahedral complex is of the form: 
wherein:
E is a donating group with a free pair of electrons; R1 is selected from the group consisting of H, alkyl, aryl and silyl; S is a solvent molecule; y is 0 or 1; and R is C, B, N, P, O, S or any other anionic bridging group.
Preferably, the tetrahedral complex is of the form: 
wherein: X is a halide; E is a donating group with a free pair of electrons; and R is C, B, N, P, O, S or any other anionic bridging group.
The process and mechanism thereof are developed further in the detailed description of the invention provided below. From the detailed description taken in conjunction with the figures and the appended claims, additional objects and advantages of the invention will be apparent to those skilled in the art.
As used herein in the specification and claims section below, the phrase xe2x80x9cstereoregularxe2x80x9d refers to a regular spatial ordering of pendant groups from the main backbone of the polymer; xe2x80x9cisotacticxe2x80x9d ordering is a stereoregular arrangement in which the pendant groups are arranged in space toward the same side of the backbone; xe2x80x9csyndiotacticxe2x80x9d ordering is a stereoregular arrangement in which the pendant groups are arranged around the backbone in an alternating form.
As used herein in the specification and claims section below, the phrases xe2x80x9celastomerxe2x80x9d, xe2x80x9celastomeric polymerxe2x80x9d and the like refer to poly alpha-olefins having elastomeric properties, including the tendency to revert to the original shape after extension. Such elastomers are obtained in various ways, including the production of polymer chains having stereosequences alternating between crystalline stereoregular (e.g., isotactic or syndiotactic) and amorphous atactic. Elastomers can also be obtained by stereoregular xe2x80x9cmistakesxe2x80x9d that are incorporated into sequences that are otherwise stereoregular. Such mistakes are often produced by complexes that exhibit high kinetic activities.
As used herein in the specification and claims section below, the term xe2x80x9csymmetryxe2x80x9d is used in accordance with the standard definition and usage: the symmetry Cn of a given molecule defines the number of times the original figure is reproduced during a complete rotation. The symmetry angle is 2xcfx80/n such that for C1, the symmetry is 2xcfx80 or 360xc2x0; for C2, the symmetry is 180xc2x0; for C3, the symmetry is 120xc2x0. Other, more specialized symmetries include C2v, which has C2 symmetry combined with vertical symmetry, and Cs which has mirror-image symmetry.
As used herein in the specification and claims section below, the phrase xe2x80x9cnatural chiral centerxe2x80x9d refers to a carbon or to a metal in which the mirror image with all the substituents is not superimposable. A carbon center is not superimposable when surrounded by four different substituents. For metal centers, however, superimposability depends on the particular symmetry of the complex. Chiral complexes, by definition, are not superimposable.
As used herein in the specification and claims section below, the phrase xe2x80x9clow pressurexe2x80x9d refers to pressures below about 1 atmosphere; the phrase xe2x80x9chigh pressurexe2x80x9d refers to pressures above about 1 atmosphere and preferably, to pressures above about 2 atmospheres. xe2x80x9cPressurexe2x80x9d typically refers to the partial pressure of the monomer species, which affects the concentration of monomer in solution. However, the total absolute pressure also influences the equilibrium between the monomer concentration in solution to the monomer concentration gas-phase.
The phrase xe2x80x9calternating pressurexe2x80x9d as used herein in the specification and claims section below, refers to a repetitive change of pressure from low to high and/or vice versa.
The phrase xe2x80x9cbidentate chelating ligandsxe2x80x9d, as used herein in the specification and claims section below, and as demonstrated in the plurality of examples provided herein, refers solely to monoanionic ligands.